The invention relates to a cooling device for an internal combustion engine, particularly a motor vehicle. A device for controlling the temperature of a cooling system of an internal combustion engine, particularly for motor vehicles, is known from German Patent Specification No. 2,806,708. This device comprises a circuit, which connects the engine to a heat exchanger, for a coolant which is circulated by means of a cooling water pump of the engine. Furthermore, it comprises a blower system having at least two blower units for the heat exchanger, which can be operated in at least two capacity ranges which are independent of the rotational speed of the engine. In addition, this device comprises several temperature switches which are arranged at different measuring points and which are associated with particular temperature thresholds. The blower motors are driven at a medium speed when a first temperature threshold is exceeded and at the maximum speed when a second temperature threshold is exceeded.
However, such a device has the disadvantage that two complete blowers (fan and engine) are required and that the blowers can only be operated at two different rotational speeds. The different temperature ranges occurring in a cooling system can only be inadequately taken into account in this manner. Furthermore, the maximum speed of the blowers becomes necessary in such devices so that the blower noise, which is felt to be disturbing at maximum speed, occurs relatively frequently.
A circuit for an electric drive motor of a fan for a radiator of a motor vehicle internal combustion engine is known from European Preliminary Published Specification No. 0,054,476, the electric motor being driven in dependence on the respective temperature of the cooling water. In this arrangement, the rotational speed of the electric motor can be influenced by means of a power semiconductor which is driven in dependence on the signal of a temperature sensor via an electronic circuit. Furthermore, a relay is provided, the switching contact of which is connected in parallel with the power semiconductor and bypasses the power semiconductor in particular operational conditions. The relay is a component of a safety circuit which is designed in such a manner that the relay is activated when the drive to the power transistor corresponds to a turn-on period of 100%. In addition, the relay is connected when the temperature sensor fails. Although the known circuit has the advantage that continuous control of the rotational speed of the electric motor or of the fan is possible, an elaborate drive system or the use of semiconductors which have very high capacity and are thus expensive is necessary.
It is not good practice to operate the power transistor with a relative operating time of more than 95% since the pulse current load capacity, particularly of metal oxide power transistors, is three to four times as high as in constantly conducting operation. In addition, a particular voltage is invariably dropped across the power transistor because of the resistance between the drain and source terminals, so that with a relative operating time of 100% of the power transistor only a rotational speed is achieved which is noticeably below the rated speed.